One of the various devices employed for measurement of ultrahigh vacuum, that is, very low gas pressure, is the Bayard-Alpert (BA) type gauge. A BA gauge operates by releasing free electrons into the low pressure space to be measured. The electrons are accelerated and collide with gas molecules present in the space, producing positive ions in the process. The positive ions are collected by an electrically conductive sensing element, an ion collector, in which the ions create a current (the "ion current"). The positively charged ions stimulate an electron flow into the collector from the external measurement circuit. Thus, the "ion current" inside the gauge gives rise to an equal and opposite electron current flowing into the collector from outside the gauge.
The ion current, in ideal conditions, is proportional to the number of ions impinging on the ion collector. Because the number of ions is also proportional to the number of gas molecules present in the space that are ionized by the injected electrons, the ion current is actually an indicator of the quality of the vacuum. The fewer gas molecules that are present, then the fewer ions are created in collisions with the free electrons, and the lower the current is in the ion collector.
As is often the case in other fields, the measurement device can produce effects that alter or limit the quality of the measurement. That is particularly so where the pressure to be measured is ultra-low (an ultrahigh vacuum), approaching a nearly total vacuum. In the case of the BA gauge, one phenomenon that presents a measurement limit is known as the "x-ray effect," which is a result of the accelerated electrons in the gauge creating not only positive ions in collisions with the gas molecules in the gauge, but also causing x-ray emissions from metals in the gauge. The x-rays cause photoelectrons to be released from conducting surfaces in the gauge. The collector current is directly affected by the photoelectron emission. An "x-ray current" is produced in the collector.
This current induces an error into the measurement, which becomes larger in proportion to the ion current as the measured gas pressures become smaller. Ultimately, the collector current caused by the x-ray effect limits the ability of the device to measure very low gas pressures.
The x-ray current is a composite of two distinct effects. The "forward x-ray effect" is caused by electrons hitting the grid of the BA gauge, producing soft x-rays. Some of these soft x-rays strike the ion collector, causing photoelectrons to be ejected from the collector. The collector current, which is positive when caused by the collection of positive ions, is increased by this electron emission. The electron emission from the collector has a similar effect as the collection of a positive ion. This enhanced forward current is known as the "forward x-ray current."
A "reverse x-ray effect" is also at work in the gauge. The x-rays created by electrons colliding with the grid strike other elements in the gauge, such as the interior of the gauge envelope. Photoelectrons are released by those elements in some of these collisions and some of these photoelectrons may strike the ion collector. These photoelectrons tend to reduce the current caused by the collection of positive ions created in electron-gas molecule collisions. Put another way, the reverse x-ray effect causes a "reverse x-ray current" in the collector.
Thus, two currents caused by the same process combine to cause error in, and at a sufficiently low pressure, to limit the sensitivity of a BA type gauge. However, the fact that the two x-ray induced currents in the gauge cause opposite currents offers the possibility of balancing the two effects, resulting in the neutralization of the x-ray effect. The result of neutralizing the x-ray effect in a BA gauge would be improved sensitivity to extremely low gas pressures and more accurate measurements of ultrahigh vacuums.